An object of the present invention is to provide a method for delivering chemical elements, which exists in a form of a solid at room temperature, as a target of a plasma for many hours consecutively, and to provide a plasma radiation source using this target.
A high-temperature and high-density laser-produced plasma (LPP) which is produced by irradiating a pulsed laser on a material is a highly brilliant radiation source covering from extreme ultraviolet (EUV) region to x-ray region. Spectral structure of the emission from a plasma depends largely on laser irradiation conditions and atomic elements in a plasma. Hence, the best target material for a plasma and laser irradiation conditions should be optimized in each application.
For example, EUV lithography (EUVL) which uses an EUV light of wavelength around 13.5 nm as an illumination light is considered the most promising lithography technology for fabricating semiconductor devices with a feature size of 45 nm and below. A plasma is the unique source for EUVL. Multilayer mirror employed in EUVL is a Mo/Si multilayer. The peak reflection wavelength of the mirror is around 13.5 nm with the reflection bandwidth of 2%. Therefore, a source for EUVL should have an appropriate spectrum matching this property of the Mo/Si mirror.
By the works started by Sugar in 1970's [non-patent reference 1] followed by O'Sullivan in 1980's [non-patent reference 2], it was made clear that 4d-4f band emission is the best to be used when a plasma is employed as a source of several % bandwidth. It was made clear that the peak wavelength of 4d-4f transitions was determined by the atomic number. The element which has the radiation peak at 13 nm was found to be tin, Sn, having the atomic number of 50. Therefore, it is obvious that the best element for an EUVL source requiring 13 nm radiation is Sn.
However, Xe continued to be the only one element as a material of the plasma in developments performed in US and Europe. Atomic number of Xe is 54 and the wavelength of 4d-4f band is around 11 nm, and the emission at around 13 nm is not strong. The reason why Xe is employed in spite of weak emission at 13 nm is the following. In EUV lithography, lifetime of an optic collecting radiation from a LPP is required to be longer than one year, i.e., more than 1E12 shots. Hence, a plasma for EUVL is required to be ultra clean. It is well known that tremendous amount of small particles of μm size, called debris, are generated when a plasma is generated on a solid plate. Debris contaminate and damage surrounding optics heavily. In mid 1990's, some methods of reducing debris, such as use of a tape target or a gas flow were tried. However, it was judged that a LPP on a solid target cannot be the source for EUVL [non-patent reference 3]. On the other hand, it was expected that damage to optics would be negligible with a Xe plasma because Xe exists as a gas at room temperature and Xe does not stick to optics. Actually, damage due to sticking is not observed for a Xe plasma.
As such, technologies using a Xe plasma has been developed. However, lately use of a Sn plasma is becoming inevitable, owing to jumping-up increase of source power requirement from a few watts several years ago to more than 100 W due to many reasons. When a Xe plasma of having a low conversion efficiency is employed, a considerably large power is required for a pumping laser. Here, CE is defined as a ratio of useable energy at 13 nm to the deposited laser energy. Then, cost of pumping laser becomes huge. Moreover, cooling of vacuum space in which a plasma is generated will become technologically very difficult. Even a tin plasma is employed by expecting a higher CE, it cannot be a source for EUVL if the debris issue is not solved. Because it was judged near ten years ago that debris issue could not be solved, a totally new idea must be devised.
(Required Minimum Mass)
First, we need to know the minimum mass to deliver. The present inventor has made a detailed theoretical consideration on a plasma source for EUVL [non-patent reference 4]. According to this consideration, the electron temperature should be 30-50 eV, the diameter should be around 500 μm, and the electron density should be around 1E20/cm3. In the case of Sn the best element for a 13 nm radiation source, the ionization degree is about 8, then, the mass required is calculated as,1×1020×(⅛)×100×(½)/203×1/(6×1023)=1.2×10−7 gand it is 0.1 μg. It is about the same mass with that of a solid Sn sphere of 30 μm in diameter.
From this calculation, in order to generate a plasma with the uniform electron density of 1E20/cm3 having several hundreds μm diameter, we need to deliver a target material of total mass equal to that of a solid sphere of several tens μm diameter.
(Mixing in a Xe Gas)
Mixing SnO2 nano-particles in a Xe gas flow was proposed by Matsui et al. [patent reference 1] expecting enhancement of 13 nm radiation. However, there are two serious problems in this proposal. The first is that particles conveyed in a gas flow cannot be confined in a small region and scatter to a wide region. When a plasma is generated, scattering of particles in a gas flow is amplified by a pressure of the plasma reaching 10,000 atmospheric pressure. Because of this, environment is heavily contaminated and surrounding material is damaged. The second is that density of particles is very low because of large scattering and that it is nearly impossible to generate a high-density plasma required for a high brilliant plasma. In short, with a method of mixing of Sn particles in a Xe gas flow, debris-freeness is very difficult to realize due to scattering of particles and enhancement of 13 nm radiation is not obtained due to low density of deliverable particle density.
(Droplet of a Solution)
Use of a droplet was also considered. When a gas target is used, gas expands after being ejected through a nozzle and the density decreases very quickly. Even in the case of a Xe LPP, performance is improved by using adiabatic cooling of the ejected gas or by using a liquefied Xe. However, when using a liquid, a liquid jet breaks up due to the growth of fluid instability, and it is not easy to extend the length of a continuous jet longer than 1 cm. Because break-up of a jet takes place randomly, droplet generation is out of control. There is a method of giving a forced vibration to the nozzle to actively control droplet generation. Once a droplet is formed, it flies long distance without breaking-up, and stable delivery of a target material becomes possible.
Use of a droplet target for a LPP generation was tried many years ago in 1973. As a means of delivering a target for laser-fusion, there was a proposal of using a solid pellet. As an alternative, Schwenn and Sigel [non-patent reference 5] proposed use of a droplet target and reported an experiment. From these previous studies, it will be obvious to ordinary experts in laser-plasma research, to use a droplet as a target of a LPP for the purpose of reducing debris. Actually in 1990's, Herz et al. [for example, non-patent reference 3] have performed x-ray generation experiments by using droplets.
It is a common knowledge among specialists in this field that x-ray wavelength depends largely on chemical elements of a plasma. A carbon plasma is employed for generation of a 3.37 nm radiation and an oxygen plasma for a 2.2 nm radiation. As a purpose of evaluating the electron density and electron temperature, Eickmans et al. [7] generated a plasma on a droplet of a water solution including LiCl or NaCl. Therefore, it is obvious for ordinary skilled researchers to use a solution including chemical elements such as Na or Mg when they need emission from a plasma having these elements. Actually, a droplet of an ethylene glycol solution in which copper nitrate is dissolved was irradiated to generate 5 to 20 keV photons emitted from a copper plasma at 1 kHz repetition rate [8]. As such, it is obvious to employ a solution such as tin nitrate or tin sulfate as a solution of droplet target because we know Sn is the best for generation of 13 nm radiation. However, there exist two problems in using a simple solution including Sn. The first problem is that a plasma of a uniform density distribution cannot be generated from a droplet. The second problem is that the source chamber is difficult to achieve high vacuum. With a single particle, a plasma of a large diameter with uniform distribution can not be generated.
FIG. 1 shows temporal change of density distribution of a plasma generated on a solid plate when irradiated by a 1 μm wavelength laser calculated using a 1-dimensopnal hydrodynamic simulation code. Material heated by absorbing a laser energy blows out into vacuum and the solid target is ablated (scraped) with a speed of the order of several tens nm/ns. However, as seen in FIG. 2, the size of the strong emission region having the density of around 3E-3 g/cm3 does not change so much. This tells us that when a diameter of a solid target is larger than several tens μm, as seen in FIG. 1, while a target becomes thinner with time, there always exists solid-state density region, a plasma of uniform density distribution is never created. The density region near critical density where emission is strong does not expand, and the emission region around 3E-3 g/cm3 stays near the initial radius of the target.
Then, in order to produce a high brilliant source with a diameter of 500 μm, the diameter of a droplet needs to be 500 μm. Because only the surface of a solid target with thickness about 1 μm is converted to a plasma, 100 times larger mass than necessary is delivered into a source chamber. This situation is not good because it increases contamination material. This material contaminates surrounding optics and causes absorption of EUV emission.
In order to keep the transmission of EUV light higher than 90%, pressure of oxygen in the source chamber needs to be lower than 0.1 Pa. When the diameter of a droplet is 500 μm and when a solvent which occupies most of the volume of a droplet is water, evaporation of solvent water produces oxygen of 5-litter volume at 0.1 Pa. An EUVL source will be required to operate at 10 kHz, and then nitrogen gas of 0.1 Pa pressure will be generated 50,000 litters in 10.000 shots in one second. Pumping this volume is an extremely heavy load to a vacuum pump. The volume of the generated gas is to be reduced to lower than 1/50. If possible, volume to be pumped is desired to be reduced to lower than 1/1,000. This requires the diameter of a droplet to be smaller than 50 μm.
[Patent Reference 1]    U.S. Pat. No. 5,991,360
[Patent Reference 2]    Japanese Patent No. 2897005
[Non-Patent Reference 1]    Sugar; Phys. Rev. B5 (1972) 1785
[Non-Patent Reference 2]    G. O'Sullivan and P. K. Carrol; J. Opt. Soc. Am. 71 (1981) 227
[Non-Patent Reference 3]    H. A. Bender, D. O'Connel and W. T. Silvast; Appli. Opt. 34 (1995) 6513
[Non-Patent Reference 4]    T. Tomie; AIST Report AIST01-A00007, “Technical consideration on a plasma for EUV lithography”, January 2001.
[Non-Patent Reference 5]    Schwenn and Sigel; J. Phys. E: Sci. Instrum. 7 (1974) 715
[Non-Patent Reference 6]    Herz et al.; Opt. Commum. 103 (1993) 105
[Non-Patent Reference 7]    Eickmans et al.; Appl. Opt. 26 (1987) 3721
[Non-Patent Reference 8]    R. J. Tomkins et al.; Rev. Sci. Instrum. 69 (1998) 3113